The present invention relates to fibers and webs comprising high glass transition temperature polymers. The webs are capable of high speed solid state deformation processing.
High glass transition temperature fibers and webs are commonly used in textile and commercial applications. The fibers typically have high tensile strength, high moduli, good heat resistance, and low shrinkage. High glass transition temperature fibers, such as poly(ethylene terephthalate) fibers, are used in many durable applications while biodegradable high glass transition temperature fibers, such as polylactic acid fibers, are used in both disposable and durable applications.
Typically, manufacturers of the high glass transition temperature fibers spin the fibers at high speeds or high draw ratios. High speed spinning causes high stress in the molten fibers which results in orientation and crystallization of molecules to near maximum levels. Alternatively, fibers may be spun at lower speed and then mechanically drawn at a high draw ratio to induce the high stress needed to create the orientation and crystallization. The high speed spinning or high draw ratio results in high performance fibers. The high performance fibers exhibit high strength, high modulus, low elongation to break, and low shrinkage.
A highly oriented and crystalline fiber has good heat resistance and dimension stability. The high speed or high draw ratio spinning can make the high performance fibers of fine denier. Therefore, these high performance fibers are widely used in the industrial and apparel industries. However, webs from these materials are not formable at high strain rates, such as occur in web post-processing, because the molecular deformation is fixed as illustrated by the high degree of orientation and crystallinity. The low elongation at break point limits the use of these fibers in post-processing such as solid state formation. Additionally, nonwoven webs of high performance fibers have been found to exhibit undue harshness that may be attributed to high tensile properties such as modulus.
An alternative to high speed or high draw ratio spinning where high stresses are generated is lower speed and lower draw ratio spinning where low to moderate stresses are generated. High glass transition polymers spun at these low speed and low draw ratio will have a high elongation at break point and may have a ductile amorphous phase. The high elongation enables the fibers processed under these conditions to be subjected to post-processing such as solid state formation. Although post-processing at high speed is possible with these fibers, the fibers have limited thermal stability which results in high heat shrinkage. If the processing temperature is raised above the glass transition temperature, fiber and web shrinkage of greater than 50% can result. Some heat treatment in constraining devices have been disclosed, for example, Ehret in U.S. Pat. No. 5,833,787, Iwasaki in U.S. Pat. No. 4,701,365, and Thompson in U.S. Pat. No. 5,958,322. The devices include tenter frames where biaxial stretch is applied, felt/drum constrainment, and forming wires with pins which constrain shrinkage.
Consequently, there is a need for the high elongation fibers spun at low to moderate speeds or low draw ratios to be thermally stable. It is desirable to provide a process which results in reduced shrinkage of the post-processable fibers.
It is also desirable to provide fibers and webs comprising high glass transition polymers that have a high elongation at high strain rates and are thermally stable to prevent excessive shrinkage. Moreover, the post processed webs will result in soft, flexible webs that are suitable for use in many industries.
The present invention relates to an intermediate web comprising of high glass transition polymer fibers. The fibers are spun at low to moderate speeds and have a relative crystallinity of from 10% to 75% of the maximum achievable crystallinity. The intermediate web is a low crystallinity web that exhibits shrinkage of more than 15% and elongation to break of more than 75% at high strain rates. This web can be heat treated to reduce shrinkage to about 15% or less, while the web is capable of at least about 60% elongation at a strain rate of at least about 50 secondxe2x88x921. Preferred high glass transition polymers include polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, poly lactic acid, and copolymers and combinations thereof. In a preferred embodiment, the fibers are comprised of bicomponent cross sectional segments with a majority section comprised of a crystallizable high glass transition polymer. The polymer has a maximum achievable absolute crystallinity of about 15% to about 60%.
The present invention also relates to processes for manufacturing a spun bond web comprising a high glass transition temperature polymer. The process has the steps of a) spinning fibers having crystallinity of from 10% to 75% of the maximum achievable crystallinity and being capable of shrinking more than 30%, b) thermally bonding the fibers using at least one calender roll which is heated above the glass transition temperature while the fibers are constrained, and c) quenching the fibers while constrained to produce a web having a web width of greater than about 70% of the prebonded web width. It is also desired that the web is heat treated to reduce shrinkage of the web to less than about 15% and crystallinity to less than about 75% of the maximum achievable crystallinity. The heat treating can occur after constrained bonding but before the quenching step, after the quenching step, before post-processing, or during post-processing. Multiple heat treatment steps may be used. Heat treatment during or after post processing may increase crystallinity as high as desired so as to enhance properties such as thermal stability. The most preferred method is to include heat treatment during post treatment immediately prior to or during molding.
Another process of the present invention is a process for manufacturing a staple fiber web comprising a high glass transition temperature polymer. The process has the steps of: a) spinning fibers having crystallinity of from 10% to 75% of the maximum achievable crystallinity and being capable of shrinking more than about 30%, b) drawing the fiber at a mechanical draw ratio of less than about 4, c) heating and drawing the fibers at a mechanical draw ratio of from 0.8 to 1.5 at a temperatures from about the glass transition temperature to about the melting point temperature for a period of time sufficient to relax internal stress of the fiber resulting in fibers having shrinkage of less than about 15% and crystallinity to less than about 75% of the maximum achievable crystallinity; and then laying the fibers into a web and bonding the web. The process for manufacturing may further comprise post-processing the web while the web is constrained.